Mayor Michael R. Bloomberg’s plan, which would limit the size of sweet drinks sold at many establishments to 16 ounces or less, exempts any beverage that contains more than 50 percent milk by volume. Officials in City Hall and in Seattle said they were unsure how those rules might affect the Starbucks family of syrupy, milkshake-style coffee drinks, catnip to thousands of caffeine-addicted New Yorkers who frequent the company’s 190 outlets in Manhattan.

“It’s hard for us to give a definitive word on which of our beverages would be impacted by the proposal,” said Linda Mills, a Starbucks spokeswoman, although she said the company was confident that many of its drinks would fall outside the proposed ban.

The Starbucks question — complicated by the varying amounts of ice, sugar and milk in each customized drink — is just one of the ambiguities facing the city as it begins a three-month public comment period on the proposed rules. On Tuesday, the Board of Health, which has final say over the rules, agreed to consider the proposal formally at its next meeting, in September.

The rules would ban large sodas sold at fast-food restaurants, movie theaters and street carts. But the Big Gulp, the supersized soda cup at 7-Eleven, would still be allowed under the proposal, because the proposal would exempt the sale of drinks in groceries or convenience stores.

Officials at the city’s health department said that they expected the plan could be tweaked ahead of the final vote, as they received feedback from companies and establishments that might be affected. In the case of Starbucks, the city plans to pore over barista training manuals to determine specific quantities of ingredients.

“This is only one of many very specific questions about very specific, unusual drinks that are likely to arise,” said Samantha Levine, a city spokeswoman.

Photo

Drs. Joel Forman, left, and Thomas A. Farley, the city’s health commissioner, at a Board of Health meeting in Queens where sugary drinks were discussed.Credit
Librado Romero/The New York Times

Members of the Board of Health, all of whom were appointed by the mayor, received their first official briefing on the plan on Tuesday, but the public was not allowed to ask questions or make comments. A public hearing on the topic has been planned for the afternoon of July 24.

Pundits and policy experts around the world have already been weighing in on the proposal. This week’s New Yorker features a film-noir-style cover that parodies the plan as a form of fizzy prohibition. And on Monday, Boris Johnson, the mayor of London, suggested on “The Daily Show” that his city might offer sanctuary to “refugees from the soda tyranny in New York,” earning loud cheers from the studio audience. (Another remark from Mr. Johnson, that Mr. Bloomberg might make a good president, was met by silence.)

While the Board of Health is widely expected to approve the plan, several of its members on Tuesday raised concerns about certain elements. Some wanted the proposal to be more rigorous — banning free refills, for instance, or doing away with the exemptions for fruit juices and milk-based drinks. One member, Bruce C. Vladeck, asked whether popcorn could be included, as well. (The city said no.)

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Sixto R. Caro, another board member, said he was concerned that poorer residents and small businesses would be disproportionately affected.

In a sign of the city’s sensitivity to public reception to the plan, members of the health panel were asked by the Bloomberg administration to avoid speaking with the public and the news media after the meeting. Howard Wolfson, a deputy mayor who guards Mr. Bloomberg’s reputation, traveled to the health department’s headquarters in Queens to speak at a news conference with the health commissioner.

A spokesman for the New York City Restaurant Association said Tuesday that the industry was considering several possible avenues of opposition.

“We believe the board is appointed by the mayor but ultimately should be accountable to the public, many of whom don’t believe in this proposal,” the spokesman, Andrew Moesel, said.

And in a television interview, Howard Schultz, the chief executive of Starbucks, said he supported Mr. Bloomberg’s objective but wondered if the plan would be effective in combating obesity.

“I’m not sure it’s the right approach,” Mr. Schultz said on “CBS This Morning.” “But we’re obviously going to follow suit and respond to him because he’s trying to do something that’s quite important.”

A version of this article appears in print on June 13, 2012, on Page A21 of the New York edition with the headline: At Starbucks, Uncertainty Over Impact of Bloomberg’s Drink Plan. Order Reprints|Today's Paper|Subscribe